I have a custom rifle in 6.5mm WSM. It makes a great long-range cartridge. I mainly use a 123 grain Lapua Scenar which I keep down to 3300 fps, but I have seen data for 140 grain bullets reaching or surpassing 3300 fps. Hodgdon RETUMBO is the go-to powder, and Federal 210 primers. 27.5" and 26" are very effective barrel lengths for this cartridge.
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Unfortunately, I haven't spent a lot of time on load development in this rifle, so I can only speak first hand about one load. 123 grain Lapua Scenar, Federal 210 primer, I have used as much as 71.5 grains of Hodgdon RETUMBO, resulting in 3350 fps with safe pressure signs.
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